1. Field of the Invention
The present invention relates to analog to digital converters.
2. Description of the Prior Art
An analog to digital converter is a circuit which converts an analog input signal to a digital representation, usually a binary number. The number of bits in the output of these circuits varies, with those circuits having the greatest number of output bits having the greatest potential accuracy.
One type of converter circuit, known as a flash converter, computes all the bits (or a group of bits) of the digital output in parallel such that the conversion is done very quickly. However, the circuitry for flash converters tends to grow very large for applications requiring a high resolution output. For example, a typical design for a flash converter having a two-bit ouput (FIG. 1) has four equal series-connected resistors and four comparators, with one input of each of the comparators connected to a corresponding resistor. A reference voltage applied across the resistors provides four distinct incremental reference voltages to the four comparators which compare these voltage levels to the analog input signal. Combinational logic converts the outputs of the comparators to a two-bit digital representation of the analog input signal.
In general, the number of comparators utilized in a flash converter increases by a factor of two for each additional bit of the output. Thus, a three-bit flash converter typically has eight comparators and a four bit flash converter has sixteen comparators. Accordingly, it is readily seen that for high accuracy applications requiring a large number of output bits, a flash converter can grow unrealizably large and complicated.
In order to reduce the complexity of the circuits, some converter circuits have utilized sequentially operated flash converter circuits. An example of such a circuit is the National Semiconductor ADC0820 8-bit high speed microprocessor compatible analog to digital (A/D) converter circuit (FIG. 2). This circuit converts a voltage input signal to an 8-bit digital representation, four bits at a time using a "half-flash" technique. The circuit has two 4-bit flash converter circuits, one of which computes the four most significant bits first The analog value of the four most significant bits is subtracted from the analog input signal to produce an analog difference or residual signal. After the subtraction, a second 4-bit flash converter computes the four least significant bits from the analog residual signal.
Each flash converter of the National Semiconductor circuit is believed to have 16 comparators for a total of 32 comparators. This is a significant savings as compared to the 256 comparators which would be required in a straightforward flash converter. However, even circuit designs such as that utilized in the National Semiconductor circuit can grow quite large if a higher resolution output is desired. For example, a 10-bit output could double the required number of comparators to 64 comparators. Accordingly, a converter circuit which has a high resolution output yet is relatively uncomplicated is needed.
Another disadvantage with previous converter circuits is that the polarity of one of the reference voltages to the circuit usually determines the polarity of the full scale input. The full scale input is defined as the magnitude and polarity of the analog input signal which produces a full scale binary output (usually all one's) of the converter circuit. For example, if the input signal ranges from a minimum of 0 volts to a maximum of positive 5 volts, a positive 5 volt reference must be used for many prior art circuits to define the full scale input as a positive 5 volts. Alternatively, a negative 5 volt full scale input often requires a negative reference voltage. In the same manner, a second reference voltage input typically defines the polarity of the low scale input which produces the minimum or low scale binary output (usually all zero's).
Accordingly, in order to properly define the input range of the converter, it is usually necessary to have one or more reference voltages of particular polarities available as inputs to the converter circuit. In many systems, the appropriate polarity reference voltage is not always readily available. Providing additional required reference voltage polarities can increase the cost and complexity of a particular system.